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What You Should Know About P and K

If you need them, don’t skimp.

When crop margins tighten, you might consider trimming phosphorus (P) and potassium (K) fertilizer applications.

It’s possible to get by for the current year and even subsequent ones with adequate or high or very high soil-test P and K levels. Here are the yield response chances on five P and K soil-test levels compiled by Antonio Mallarino, Iowa State University (ISU) soil scientist.

  • Very low: 80%
  • Low: 65%
  • Optimal: 25%
  • High: 5%
  • Very high: <1%

“With optimal P and K soil-test levels, there is not that much return with $3 corn,” says Mallarino. On high-testing soils, the return is even less likely, he adds.

READ MORE: 5 factors to consider for potassium fertilizer

Soil Tests Are Key

On very low or low-testing soils, though, it’s a different story. Together with nitrogen, P and K form the Big Three of crop nutrients.

P plays a crucial role in plant processes like photosynthesis, respiration, and energy storage and transfer. K benefits include increased root growth, improved drought tolerance, and enhanced photosynthesis.

The only way you’ll know your levels is by testing them.

“Soil tests for P and K are not perfect, but they are useful, especially with unfavorable commodity prices,” says Mallarino. “Soil tests are a very small investment that can save a lot of money.”
Accurate soil-sampling techniques are needed to key the best results.

Furrowed brows and a that-can’t-be look can accompany a P and K analysis.

“People will say about suspected erroneous P and K levels, ‘This lab must be off,’ ” says Mallarino.

That’s not often the case. Instead, questionable readings often result from insufficient numbers of samples or cores per sample caused by frequent high spatial variation. One way to improve on classic sampling by soil-type method includes either grid sampling or zone sampling.

“The problem with soil sampling by soil type is there can be a high variation of P and K in a field due to years of manure and fertilizer applications,” says Mallarino.

Variation can also exist with zone sampling, although it is less than with sampling by soil type.

Grid sampling yields the most accurate P and K soil-test level results. It’s more costly, though, since more samples are required, says Mallarino. “It describes P, K, and pH variation better, though,” he says. “It can be used with yield maps to estimate P and K removal. It adapts so well to variable-rate application.”

LISTEN: Managing soils for phosphorus

Efficacy Index

A Sampling Efficacy Index examines the capacity of a sampling method used to identify field areas with different response to P or K. The higher the percentage, the more accurate it is. Here’s how various sampling methods stack up based on ISU analysis.

  • 0.3- to 0.5-acre grids: 100% (base)
  • 2.5-acre grids: 50%
  • Zone sampling: 39%
  • Soil-type sampling: 22%

Grid sampling also fits well with variable-rate application for P and K. “Some farmers will think, ‘This is my pool of money and I’m going to divide it across the farm,’ ” says Mallarino. However, variation of P and K levels between or even within fields make variable-rate technology a good fit for applications of these immobile nutrients.

Proper Sampling

For accurate results, though, proper grid sampling must occur.
“You have to have more than four to five cores per sample,” he says. “You also have to sample in 2.5- to 5-acre grids.”
If not, zone sampling or a combination of both is recommended. “I wouldn’t do grid sampling if grids are larger than approximately 5 acres,” says Mallarino. “A good zone sampling approach may be better.”

Sampling depth is crucial – 6 inches is best for P and K in Iowa, regardless of the tillage system, says Mallarino.

“It is not better to sample any shallower or deeper,” he says. “Sampling more than one depth is more expensive and isn’t useful.”

Yearly testing for P and K isn’t required, but it is advised every other year.

“Sampling every two years for P and K is a good investment, mainly when significant field areas test low. It prevents having to guess when sampling every four or more years,” says Mallarino.

Should You Lime?

Optimum soil pH is essential to key proper nutrient cycling, soil microbial activity, and soil structure. That’s why you need to check your field’s soil pH. If your soil pH is 6.5 or above, don’t worry. That’s a sufficient pH level for all crops, says Mallarino.

If you farm soils with high pH subsoils (calcareous), liming to raise soil pH to 6.0 for corn and soybeans is sufficient. Below those levels, though, lime is recommended.

“If your soil’s pH is 5.4 or 5.5, you need to apply lime, even with low crop prices,” says Mallarino.

 On the flip side, though, corn and soybean yield decreases can even result when soil pH is raised up to 7 to 7.4, he says.

READ MORE: Fertilizing crops in tough times

Band or Broadcast?

If you’re applying P and K, banding is seldom more efficient than broadcasting in Iowa soils.

“Other than for starter, there is no economic advantage for banding of P under tillage, ridge-till, no-till, or strip-till,” saysMallarino.

Reducing the ISU-recommended P rates by banding on low-testing soils is not a good idea, as it may reduce yields and profits, says Mallarino. It can also increase future fertilization needs. One perk: Subsurface P applications may reduce exposure for P losses from surface runoff and water-quality impairment.

Potassium is a different story, since research in Iowa fields shows deep-banding K at 5 to 6 inches deep is beneficial on ridge-till, but it is only sometimes beneficial under no-till or strip-till, says Mallarino.

Cutting P or K application rates for low-testing soils is not recommended, according to ISU soil-test interpretations, since it may cut yield and profit, says Mallarino. Under some conditions, a small amount of starter fertilizer containing P applied to the corn seed furrow or below and beside seeds can complement a primary broadcast application, he says. Starter K benefits are less frequent, he adds.

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